Titanium occurs in nature in many mineral forms. The most common minerals and those to which most of the conventional beneficiation and purification processes are directed are rutile, ilmenite and leucoxene ores.
Increased demands for rutile [TiO.sub.2 ], which is not an abundant mineral, have presaged the development of efficient processes for upgarding low grade titanium ores such as ilmenite to produce products with characteristics similar to natural rutile, i.e., the so-called synthetic rutile containing more than 90% TiO.sub.2.
In ilmenite and other common titanium ores, the titanium exists chemically combined in true chemical compound form as FeO.TiO.sub.2. All of the conventionally employed upgrading methods require a heat treatment followed by an acid leach to reduce the compound and to remove impurities. Typical of such processes are those described in U.S. Pat. Nos. 3,252,787 (Shiah); 3,257,198 (Volk et al.); 3,784,670 (Yamada et al.); and 3,926,615 (Leilach et al.).
According to these patented processes, the ilmenite is first reduced at very high temperatures (about 1000.degree. C.), sometimes under pressure, and in the presence of strong reducing agents. The purpose of the combination of high temperature and reducing agent is to break up the crystal lattice and composition of the ilmenite into TiO.sub.2 and ferrous oxide and/or metallic iron. The latter are generally removed by various leaching steps leaving a rutile having a high content of TiO.sub.2.
In the Leilach et al. patent a brookite type of material is obtained by the high temperature oxidation of ilmenite. Special precautions are taken during the heating step to avoid the formation of hematite crystals from the iron which is released from the ilmenite lattice during heating.
In about 1968 a new type of titanium ore, since named anatase, was discovered in Brazil in complex deposits known as alkaline pipes. In these deposits the titanium exists as TiO.sub.2 in complex association with other minerals such as perovskite, various phosphates, limonite, magnetite, hematite, etc. It is to be understood that the titanium in anatase exists as a true oxide (TiO.sub.2), although in association with other compounds, and not as a titanate (FeO.TiO.sub.2) as in ilmenite. A typical analysis of anatase ore is as follows:
TiO.sub.2 --28% PA1 Fe--38% PA1 CaO--1% PA1 MgO--&lt;1% PA1 P.sub.2 O.sub.5 --2% PA1 SiO.sub.2 --1% PA1 Anatase--18% PA1 Hematite--25% PA1 Ilmenite--2% PA1 Limonite--2% PA1 Apatite--&lt;1% PA1 TiO.sub.2 --83% PA1 P.sub.2 O.sub.5 --1% PA1 Fe--5% PA1 Ca--&lt;1% PA1 SiO.sub.2 --&lt;1% PA1 Anatase--84% PA1 Hematite--4% PA1 Silicates--1% PA1 Phosphates--&lt;1% PA1 (a) If the iron oxide is attached to the anatase crystal, the physical change in dimension releases the iron impurity from the anatase crystal. PA1 (b) If the original limonite was included in the anatase crystal, the formation of a magnetite crystal larger than the original limonite one, will cause the anatase crystal to strain and break apart or develop cracks. In either case the original iron impurity can be removed by subsequent treatment, physical or chemical.
A typical petrographic analysis for the ore is as follows:
Anatase is much more reactive during chlorination than the rutile form of TiO.sub.2 due to the differences in crystal structure. It is desirable to maintain this reactivity of the anatase during processing of the ore.
Some of the contaminants in the ore may be removed by conventional mechanical beneficiation processes whereas others require more elaborate processing. Thus, by a combination of steps involving desliming, controlled grinding and magnetic separation, it is possible to obtain concentrates with TiO.sub.2 contents of about 60-65%. If a flotation step is added, which requires a smaller particle size, mechanical concentrates of up to 80% TiO.sub.2 can be obtained. A typical chemical and petrographic analysis of a mechanical concentrate without flotation is as follows:
TYPICAL PETROGRAPHIC ANALYSIS OF CONCENTRATE:
The latter is preferred over the concentrate obtained by flotation because it retains a coarser particle size distribution which is required in the industrial chloride process for manufacturing TiO.sub.2 pigments, titanium sponge, etc.
Innumerable research efforts have been aimed at obtaining concentrates with a high content of TiO.sub.2, starting with minerals other than ilmenite, in which the titanium is in the form of iron titanate. Attempts made so far have proved fruitless, taking as the starting point either minerals in which the titanium is in the form of titanates other than of iron, such as perovskite and titanite (sphene), or minerals in which the titanium is in the form of oxides, such as anathasium and brukite, in the natural form. With all of these experimental techniques, however, even when high contents of TiO.sub.2 are obtained, the impossibility of removing impurities strongly associated with titanium did not permit the obtention of a final product suitable for marketing. The prohibitive prices of the attempted processes, in turn, discouraged any attempts at proceeding with studies in this respect.
Strict anti-pollution legislation in the developed and developing countries also created a greater demand for concentrates of minerals with a high content of titanium for the manufacture of pigments, a sector of the industry responsible for the consumption of about 75% of the titanium minerals produced in the world. This tendency is aimed at reducing or even eliminating the production of pollutant effluents, such as occurs mainly in the process of obtaining TiO.sub.2 by the sulphate process using ilmenite as the raw material.
In the process for concentration of minerals as presently known, the main disadvantages are the need for using raw materials that meet certain specifications as regards content of impurities, especially phosphorus; the high temperatures at which reduction takes place; and the considerable consumption of leaching solution due to the low content of titanium dioxide (TiO.sub.2) in the raw material.
The above-described mechanical concentrates of anatase are suitable for use in some methods for preparing pigments, etc. However, the presence of impurities such as phosphorous disqualifies them for use in the sulfate process. This last disadvantage has been circumvented with the recovery of the leaching solutions, but, even so, this approach calls for greater investment with consequent increases in the price of the concentrate so obtained.
The presence of alkaline and alkaline earth elements disqualifies them, in turn, for use in the manufacture of pigments by the chloride process, inasmuch as its chlorides, which have melting points in the range of operation of the chlorination reactors and do not volatilize within the latter, jeopardize the fluidized bed normally used in this process, or form residual products that are extremely detrimental to the operation of the equipment.
It is an object of the present invention to provide an efficient and inexpensive method for upgrading anatase or titanium ores or concentrates of similar composition.